As illustrated in FIG. 1, a typical prior-art snowmobile 10 has a chassis or frame 12 and a motor 14 mounted to the chassis. The motor 14 may be either a two-stroke or four-stroke internal combustion engine. The motor 14 is connected via a power-transmission system (not shown) to an endless track 30 which is mounted around a rear suspension system 28. Handlebars 16 are provided for steering the snowmobile. The handlebars 16 are linked via a steering linkage assembly (not shown in FIG. 1) to a pair of front skis 18. Each front ski 18 is connected to a front suspension system 20. The front suspension system 20 includes a swing arm 24 on each side of the snowmobile 10. The swing arm 24 is connected to the chassis 12 and to the front suspension system 20. The front suspension system 20 also includes a shock absorber assembly 26 for each front ski 18.
There are two types of front suspension systems that are commonly used on snowmobiles. The first type is known as the swing-arm type front suspension. The second type is known as the double A-arm front suspension.
As illustrated in FIG. 2, the swing-arm-type suspension system has a swing arm 24. The swing arm 24 is pivotally mounted to the chassis 12 of the snowmobile. A rear portion of the swing arm 24 has a pivot collar 58 which pivotally connects to a chassis pin 60 which protrudes transversely from a side portion of the chassis. A front portion of the swing arm 24 has a tubular, cylindrical spindle holder 56 whose inner diameter is dimensioned to receive a spindle 34. A plurality of splines 72 is located at the top of the spindle 34. The splines engage with a star-shaped aperture in a steering arm 74. The steering arm 74 is connected via a steering linkage assembly 76 to a steering column (not shown). Also affixed to the cylindrical spindle holder 56 (at the front portion of the swing arm 24) is a bracket 62. The bracket 62 fastens to a bottom rod eye of a shock absorber 26 while the top of the shock absorber is fastened to the chassis 12. The bracket 62 also has attachment points for an upper suspension arm 64 and a lower suspension arm 66. The upper suspension arm 64 and the lower suspension arm 66 are both pivotally connected to the chassis 12 and are also pivotally connected to the bracket 62. The upper suspension arm pivots about an axis that is parallel to the centerline 68 of the snowmobile. Likewise, the lower suspension arm pivots about an axis that is parallel to the centerline 68 of the snowmobile.
Still referring to FIG. 2, the spindle 34 connected to the front ski 18 via a bridge-shaped bracket 36. The bridge-shaped bracket 36 is an elongated U-shaped structure with a bottom portion 38 connected to two upwardly extending side portions 40, 42. The bottom portion 38 of the bridge-shaped bracket 36 is adapted to fit onto the front ski 18. The bridge-shaped bracket 36 has four holes that receive four respective threaded studs 44 that extend upward from a runner 32 that lies underneath the front ski 18. Nuts 46 are tightened onto the threaded studs to fasten the bridge-shaped bracket 36 to the front ski 18. Each side portion 40, 42 of the bridge-shaped bracket 36 has a hole 48 through which a spacer 50 may be inserted. There are corresponding holes on the sides of the pivoting bracket 31a. The spacer 50 is tubular so that a bolt 52 may be inserted inside the spacer. The bottom portion of the spindle 34 is welded to a pivoting bracket 31a which pivots on a bottom bracket 31b. The bottom bracket 31b is affixed to the front ski 18. The pivoting bracket 31a is pivotally fastened to the bridge-shaped bracket 36 by the bolt 52, spacer 50 and nut 54. When the spacer 50 is inserted in the holes 48 and through the corresponding holes of the pivoting bracket 31a, the spacer 50 restrains the pivoting bracket 31a. The pivoting bracket and the spindle 34 may then rotate in a semi-circular recess located in the bottom bracket 31b. This permits the spindle 34 to pivot with respect to the front ski 18.
In operation, when the snowmobile 10 encounters a bump, the swing arm 24 will rotate about the chassis pin 60. The upper suspension arm 64 and the lower suspension arm 66 act to constrain the motion of the swing arm 24 while the shock absorber assembly 26 acts to dampen the motion by absorbing and dissipating energy. As the swing arm moves, the spindle 34 will pivot with respect to the front ski 18 by virtue of the brackets 31a, 31b. 
One shortcoming of the swing-arm suspension system is that it is quite heavy. Another shortcoming of the swing-arm suspension system is that the swing arm is oriented at a downward angle relative to the horizontal plane. This downward angle is necessary to provide sufficient ground clearance for the front of the vehicle. Since the swing arm is angled downwardly, the spindle and the front ski must first travel in the forward direction when the front ski encounters a bump. In other words, in order for the front suspension system to compress, the front ski must move forward before it can move upward. Since the vehicle is usually moving forward when it encounters a bump, this forward movement of the front ski augments the impact on the chassis and thus results in rider discomfort.
As noted above, a second type of front suspension system that is well known in the art is the double A-arm suspension. As illustrated in FIG. 3, a double A-arm front suspension system 20 has an upper A-arm assembly 21 and a lower A-arm assembly 22. The upper and lower A-arm assemblies 21, 22 work together to ensure that the front ski 18 and the spindle 34 move vertically when the front ski 18 encounters a bump. Unlike the swing-arm suspension, the front ski 18 and spindle 34 do not have to move forward in order to move upward. Thus, rider comfort is improved vis-à-vis the swing-arm design. However, the double A-arm design is also heavy. Furthermore, the double A-arm design is costly to manufacture because of the number of parts and their complexity.
There is therefore a need for an improved front suspension system that is lighter than the prior art suspensions and that is less expensive and easier to construct.